1. Field of the Invention
The present invention relates to a small cylindrical reformer, suitable for use in the production of hydrogen from fossil fuels for a small fuel cell power plant, in which a reforming reactor, a water gas shift reactor (high temperature or low temperature), a combustor, a heat exchanger, a steam generator, etc., may be integrated in a single container to form an optimal heat exchanging network, minimize heat loss and optimize heat exchange efficiency, whereby the reformer is designed to be easily processed and manufactured and to have a compact size, resulting in high efficiency, light weight and ease of mass production.
2. Description of the Related Art
Presently, as the type of fuel to be consumed is gradually moving toward clean fuel having a high hydrogen/carbon-atom ratio and a low pollution level, an industrial society based on hydrogen as a harmless clean fuel is expected to arise quickly. In industrial fields, hydrogen has already been applied to ammonia synthesis, methanol synthesis, petroleum refining (dehydrogenation, hydrogenation, etc.), general and fine chemical industries, electronic and semiconductor industries, food and metal processing industries, etc. Further, in the energy fields, hydrogen is used as the fuel of fuel cells for home or power plants and of power supply sources for fuel cell automobiles, being able to solve at one time problems, related to a propellant, electric power supply and energy efficiency of a space shuttle, in which an internal combustion engine is not used, and to environmental pollution, at one time.
Presently available methods of preparing hydrogen include, for example, steam reforming, partial oxidation or autothermal reforming of fossil fuel (coal, petroleum, natural gas, propane, butane), and water electrolysis. Although hydrogen may be obtained from a by-product gas of a petroleum refining process, the steam reforming method is regarded as an economical application technique which is most widely used commercially.
In the case where the steam reforming method is applied to produce hydrogen on a large scale, a reforming reactor may be operated at a high pressure (15-25 bar) and high temperature (850° C. or higher). Thereby, hydrogen may be desirably produced, but the total efficiency thereof is low. In addition, when producing hydrogen on a small scale, the installation cost may be drastically increased. In addition, the apparatus may be enlarged for stable operation, and each process may be separately conducted, thus it is difficult to increase heat efficiency due to the limitation in combining heat.
Hence, in order to solve the above drawbacks of reformers for home or commercial small fuel cell systems, attempts have been made to combine each process, develop a catalyst suitable for a small fuel cell system, optimize the reformer using a heat flow analysis, simplify the structure of the reformer to increase processability and productivity, and integrate constituents of the reformer to decrease the size thereof, so as to reduce initial start-up time and heat loss and increase heat efficiency.
In this regard, U.S. Pat. No. 5,932,181 discloses a hydrogen generator using a cylindrical combustion catalyst, which comprises a desulfurization reactor, a reforming reactor, a water gas shift reactor, a pressure swing absorption unit, etc. This patent is advantageous because the number of parts of equipment is decreased so that the space necessary to set it up may be reduced to miniaturize the generator, and the manufacturing cost is drastically decreased, and also efficiency and start-up time may be improved by heat exchanging the desulfurization reactor. In the above generator, heat generated from the desulfurization reactor, the reforming reactor, the steam generator and the combustor may be combined together, however heat generated from the water gas shift reactor and the subsequent parts cannot be combined, thus decreasing the total heat efficiency and increasing the size of the device.
In addition, U.S. Pat. No. 6,117,578 discloses a reformer for fuel cell power plants using two heat supply sources of a combustion catalyst or a burner, in which a combustion catalyst and a reforming catalyst are applied on the wall in the reformer to maximize catalyst efficiency, and thus, the reformer may be operated even at a low temperature. Thereby, reaction operation conditions become mild. However, high heat efficiency is difficult to obtain, due to the lack of combination of post processes.
Korean Patent Laid-open Publication No. 2002-82061 discloses a compact steam reformer using a cylindrical metal fiber burner, in which a reactor, a heat exchanger and a steam generator may be integrated, realizing the compact steam reformer. Further, as the heat supply source necessary for reforming, the cylindrical metal fiber burner is used for uniform heating, whereby the temperature gradient of the catalyst layer in the reforming reactor may be reduced to maximize reforming efficiency. Also, since steam is generated using a combustion exhaust gas, energy efficiency increases, and an additional steam generator is not needed, resulting in a highly simplified system and decreased device and operation costs. However, in the above reformer, a heat streaming is not optimized, and the exhaust gas stream should be accurately controlled during operation. Further, excessive fuel is consumed, and heat generated from the water gas shift reactor and the preferential oxidation reactor is not recovered, decreasing the heat efficiency. Furthermore, heat exchange between the exhaust gas and the steam results in an undesirably enlarged reformer.
Recently, U.S. Pat. No. 6,481,207 discloses a cylinder type reformer for use in maximizing heat efficiency, in which a burner is provided at the central upper portion of the reformer, and a plurality of cylinders are sequentially radially disposed from the highest temperature region to the lowest temperature region to maximally suppress heat loss. In addition, the cylinders are attempted to be integrated while increasing heat efficiency by conducting heat exchange therebetween. The reformer is designed in such a way that the reactor is axially positioned so as to enable the cylinder type reforming reactor to resist thermal deformation. In the above reformer, a preheating layer and a heat recovery layer are provided for effective heat exchange. However, there is no means for preheating air, and also many spaces for liquid stagnation are present in the structure, and hence, problems may occur when water is not completely evaporated or the product is condensed. Further, the above reformer is not easy to manufacture due to its complicated structure.